The Oklo natural reactors: cumulative fission yields and nuclear characteristics of Reactor Zone 9

Abstract

The isotopic composition of molybdenum, ruthenium, palladium, silver, cadmium, tin, tellurium, neodymium and uranium have been measured by solid source mass spectrometry in eight uraninite samples from Reactor Zone 9 at the Oklo natural reactors. Cumulative fission yields have been derived for most of these elements, after correcting where necessary for the primordial component of the element concerned. Neutron capture reactions on a number of nuclides with significant thermal cross sections, and fission chains in which one of the precursor nuclides has a lengthy half-life, have also been examined to provide information on the relative mobilities of the elements involved. The Pd fission yields have been utilised to calculate the proportions of 235U, 238U and 239Pu fission to be approximately 88%, 8% and 4% respectively in Reactor Zone 9. It has also been shown that nearly 50% of the fissioning 235U nuclides were produced from the α-decay of 239Pu. The integrated neutron flux in this zone was calculated to be approximately 3.6 × 10 20 n cm −2. Estimates of the spectral index show that approximately two-thirds of the neutrons in Zone 9 were thermalised. Although it is unlikely that the Oklo reactors operated continuously, this study has shown that the duration of criticality in Zone 9 was approximately 2.2 × 10 5 years. During this period the average fission density was 0.92 × 10 20 fissions cm −3, which represent a total energy output of 4 × 10 8 J g −1 of sample with an average power output of 8.1 × 10 −5 W g −1 of sample. The accurate analysis of the isotopic abundances of nine elements, has enabled the nuclear characteristics of Reactor Zone 9 to be established, thus providing a base for estimating the nuclear inventory of the various fission products. This will allow a comparison to be made with the present-day elemental abundances and hence an evaluation of the mobility of these elements in the geological environment at Oklo.